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COVID-19 ISO Insights

Aerosol Transmission of the COVID-19 Virus Concerns Linger

July 20, 2020

By: Christopher Sirota, CPCU

Some very small things are apparently a very big deal to a growing number of disease experts around the world. Recently, these experts have reportedly been expressing concerns about how much novel coronavirus can be found floating in the air within so-called aerosols—the very tiny particles of saliva, mucus and water that everyone exhales when, for example, talking, singing, coughing and sneezing.

We previously posted that there is reportedly a risk that the novel coronavirus (SARS-CoV-2), which causes COVID-19, may be transmitted through the air as an aerosol emitted from an infected person. For example, an April 2020 National Health Institute report regarding healthcare worker exposure to SARS-CoV-2 noted that "[s]neezing and coughing are effective aerosol generators, but all forms of expiration produce particles across a range of sizes."

The novel coronavirus may even be able to hang around in stagnant air for perhaps hours, according to a page dedicated to the topic in the New York Times, but experts point out that there is still little understanding of how often disease transmission might occur in this way for the current virus.

Nonetheless, more material appears to be surfacing that supports the possibility of COVID-19 being transmitted through aerosols.

Letter to World Health Organization From Over 200 Scientists Relays Concerns About Aerosols

According to a separate New York Times article, a letter signed by over 200 scientists from over 30 countries has urged the World Health Organization (WHO) to alter their recommendations to address potential aerosol transmissions of the novel coronavirus.

According to the article, as of the letter being sent, the WHO recommends the use of N95 masks and proper ventilation to mitigate airborne transmission "only after medical procedures that produce aerosols, or droplets smaller than 5 microns […]." The WHO infection control experts reportedly consider the evidence to support aerosol transmission in other situations as inadequate, and the WHO continues to recommend handwashing as a key method to mitigate the transmission of the virus based partly on their understanding that fomites—objects contaminated with the virus—are often the cause of transmission.

The letter from the scientists, however, reportedly offered a different collective expert opinion: that evidence for aerosol transmission is mounting (Full letter available here). The Times article included two examples of purported aerosol transmission, one that occurred at a restaurant, and another that occurred at a German meat processor.

Per the Times, the WHO may be hesitant to include recommendations to address this new risk pathway because of the potential for increased panic and increased costs to prevent aerosol transmission.

Noting various potential precautions, the letter lists the following that could be recommended:

  • "Provide sufficient and effective ventilation (supply clean outdoor air, minimize recirculating air) particularly in public buildings, workplace environments, schools, hospitals, and aged care homes.
  • Supplement general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.
  • Avoid overcrowding, particularly in public transport and public buildings."

(See our related post on modeling indoor airflow here)

In response to the letter, Nature subsequently reported that, during a July 7th WHO press conference, the organization suggested plans to "[…] issue new guidelines about transmission in settings with close contact and poor ventilation." With that said, Nature highlighted that the WHO still wanted “to see randomized trials that demonstrate that interventions aimed at controlling aerosols actually work. One example […] would be a trial showing that tight-fitting respirator masks offer better protection than looser-fitting medical masks in a health-care setting.”

Some Studies Look At How Aerosols Move Through the Air

Regarding the mounting evidence to support aerosol transmission, one pre-print study was published in in April 2020 that examined air and surface samples around 13 patients that were pulled from the Diamond Princess cruise ship. Another, published in June 2020, included comparisons of aerosols produced when wearing different types of masks. Additionally, according to the June 2020 study, experts have explored the potential differences in confined environments in which aerosols might be encountered, such as airplanes and cars.

Here are some highlights by confined space:


  • "[In a airplane] complete cabin air exchange [happens] every two to 3 min (20–30 air changes per hour (ACH)) […] In a typical aircraft, the recirculated air is passed through high-efficiency particulate air (HEPA) filters, with which in excess of 99.97% of particles characterized by aero-diameter of 0.3μm could be removed from the ingress of cabin air."
  • wearing masks can reportedly reduce the cough jet distance, for example from reaching as far as five seats ahead, to one seat ahead.
  • "the environmental factors such as moderately low relative humidity (50%), low temperature (< 25°C), and moderate ACH (< 30 per hour) would set the platform for the SARS-CoV-2 to sustain for extended periods within the cabin."


  • "[…] with the air-conditioning (AC) system off, the ACH for a stationary vehicle was in the range of 0.42–1.09 per hour. With the AC on, ACH was between 1.96 and 3.23 per hour, and with the AC off and the fans on, it varied in the range of 8.7–10.7 per hour."
  • "With the windows closed and the fan set on fresh air, the ACH was between 13.3 and 26.1 per hour"
  • "letting the vehicle move with an average speed of 29 km/h […] with the window open and the ventilation system off, [there was] an ACH of 71 per hour […]"
  • Reported Case: a COVID-19 infected backseat passenger reportedly infected a driver after driving with the windows closed and AC on, for no more than one hour.
  • Reported Case: a person was reportedly infected from an asymptomatic person after driving for more than 15 minutes with the windows closed and AC on.
  • "When two scenarios (Scenario 1: AC on and windows closed; Scenario 2: AC off and windows opened) are critically reviewed, one can speculate that the scenario 2 will be better in controlling the SARS-CoV-2 virus; hence strongly recommended at least until the COVID-19 pandemic ceases."
  • Reported case: a family reportedly avoided contracting COVID-19 by driving a symptomatic family member sitting in the backseat for 30 minutes to the hospital with the windows open.
  • A study showed that within the car cabin "…droplets do not travel far and [remain] confined to a limited space (even not beyond the driver's seat), but [a] cloud of aerosol will drift far…"
  • A study showed that cough jet distances can be reduced by wearing a variety of mask types within a car cabin.

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